Pneumatic Systems

Muhajir Ab. RahimSchool of Mechatronic Engineering

Pressure

• Pressure is the force or push exerted upon a surface. • Air consists of billions and billions of air molecules that

are in constant motion. These air molecules are moving and constantly bouncing off of any surface they encounter.

• It is the sum of all these impacts on a surface added up together that equals pressure

• If we blow up a balloon, the balloon will expand because there are more air molecules bouncing off of the inside of the balloon than there are bouncing off of the outside of the balloon. (see Fig. 1)

• If we blow more air into the balloon, there are even more air molecules bouncing off the inside of the balloon and the balloon

gets even bigger because the pressure pushing against the inside surface of the balloon is greater than the pressure pushing on its

outside surface (see Fig. 2)

• If we could take the balloon above sea level we would find that the balloon would increase in size. Since there is less pressure on the outside to counteract the pressure on the inside, the balloon will expand and get bigger until eventually it stretches so much it will burst. (see Fig. 3)

How to increase air pressure?

• Compressing the air

• Heating the air

“when pressure is higher, its temperature will also be higher, because air that is compressed will become very hot”

• If we take a cylinder with a piston, as in Figure 4A, and push the piston into the cylinder as in Figure 4B, we compress the air into a smaller space. Since the air molecules have less space to move in they bounce off the cylinder walls more frequently causing an increase in pressure.

• In Figure 5B heating the air causes air molecules to move much faster. Since they move faster they bounce off the walls of the containers  more often resulting in an increase in pressure. 

Humidity• Humidity is the measurement of the amount of water vapor or

moisture contained in the air.  • Hot air has the capacity to hold a great deal of water vapor or

humidity.  Because the air molecules are hot, they move around very fast and keep the water vapor from forming into droplets. 

• As this hot air is cooled, the air molecules move with less velocity and are closer together.  This allows the water vapor to form into droplets. 

• The temperature at which the water vapor in the air begins to condense and form droplets is called dew point. 

• On a hot humid day, a glass of ice water will cool the air around it to below the dew point, causing the water vapor in the air to condense on the glass in  the form of droplets. 

• On a hot day with very little humidity in the air the outside of the glass will barely become damp.

• In a pneumatic system the act of compressing the air causes it to become very hot. 

• This hot air has the capacity to hold a large amount of water vapor. 

• As compressed air is used or released from compression it becomes very cool. 

• Many times it will cool off below the dew point causing any water vapor to form droplets in the air line. 

• The act of compressing and releasing the air from compression can produce the undesirable effect of water in an air line

Flow• Flow is the measurement of how much air is being used, measured in

Cubic Feet per Minute (CFM).  • The flow of air through an air line or out of an orifice is measured in

CFM or Standard Cubic Feet per Minute (SCFM).  • There is an important difference between pressure and flow.  • Pressure is the push that causes the flow.  If we blow up a balloon

and tie it off, as in Figure 1, we cause pressure.  • If we untie the balloon and let the air escape we cause flow.  • Pressure causes force.  Flow causes movement.

The amount of air that is discharged from an orifice or tube

is dependant on basically two factors:

1. The size of the orifice, and

2. The amount of pressure in the pneumatic system pushing the air out of the orifice.

FRL

• Filters, regulators, and lubricators are generally used together on each machine to condition the air properly. When used together they are referred to as FRL

• In the pneumatic system, the air flows through the filter first, the regulator second, and, if needed, the lubricator third.

Filters

• Filters are usually used in conjunction with a separator bowl, see Figure 2.

• The air enters the bowl where it will swirl around the filter, forcing contaminates and water to accumulate in the bottom of the bowl.

• The air then passes through the filter and exits through the output port.

• Filter separators are stamped with an arrow indicating which direction the air must flow through them, aiding in proper installation.

• Filters serve an important function by removing dirt and contaminants from air that can prematurely wear moving parts.

• Therefore, filters must be constantly maintained to avoid clogging.

• A clogged filter can restrict the air flow, causing a machine to work sluggishly. Reusable filters should be periodically cleaned.

• Disposable filters should be replaced when dirty, and the separator bowl should be washed out with a mild detergent. 

• Caution should be used when using cleaning solvents to avoid weakening or destroying polycarbonate bowls.

• Figure 3A shows a filter separator with a manual drain. Depressing the small button at the bottom allows excess water to drain.

• Figure 3B shows a filter separator with an automatic drain. When the water level reaches a certain point, a float will rise and allow water to drain until the float is no longer supported and drops back into place.

• The automatic drain has the advantage of less maintenance, but care should be taken to see that the filter is properly serviced.

Regulators

• Many times the air pressure required by a machine is lower than the air pressure supplied by the compressor.

• In this situation, a regulator is placed in the air line to reduce or regulate the pressure to the level required by the machine

• In Figure 4A, the regulator's poppet is against its seat and allows no air to pass through the regulator.

• As the handle is turned, Figure 4B, the spring pushes down on the diaphragm which pushes the poppet down off its seat. Air can now flow through to the machine.

• As air flows into the air line going to the machine, pressure builds up in the air line. As the back  pressure increases, it pushes up on the diaphragm.

• Eventually the back pressure will overcome the spring pressure, Figure 4C, and pushes the diaphragm up, pushing the poppet up against its seat, stopping the air flow.

• Thus we can regulate or reduce the pressure to a level usable by the machine. • As the machine is operated and air is used, the back pressure will be reduced. • This allows the spring to push the diaphragm and poppet down, letting more air

through the regulator until the back pressure pushes the diaphragm and poppet up again, shutting off the air flow

Lubricators• A great demand is placed on the

moving components of a pneumatic system. Spool valves and air cylinders are expected to last for hundreds of thousands of cycles. To assure this life expectancy it is imperative that they be properly lubricated.

• As air flows through the lubricator, see Figure 6, oil is forced up a tube where it is metered.

• Eventually the oil will form a drop on the end of the tube and is dropped into the air flow.

• The air flowing through the air line will then carry or push this oil to the moving components downstream.

• If too much oil is introduced into the air line, valves and cylinders may become sluggish and sticky, resulting in erratic operation.

• Not enough oil will allow moisture to rust and corrode metal parts and fittings, and cause the parts to wear rapidly.

• Manufacturers' adjusting specifications should be closely followed for proper machine operation.

Schematic Symbols

• A diamond shape is used to indicate a device which conditions the air.

• By adding other symbols to the diamond we can tell if it is a lubricator, a filter or some other type of conditioning device.

• Figure 1A shows the symbol for a filter separator with a manual drain. • The dotted line extending from the top of the diamond towards the bottom

indicates a filter which removes dirt and contaminants from the air. • The straight line across the bottom of the diamond indicates a separator

where water and other contaminants can accumulate. • The horizontal lines on the left and right side of the diamond indicate the

input and output ports respectively. • The line coming straight out the bottom of the diamond indicates a drain.

This drain is manually operated and is used to drain the water from the bowl.

• Figure 1B is the symbol for a filter separator with an automatic drain. • The V symbol added near the bottom of the diamond indicates the drain

operates automatically. When the water reaches a certain level it will automatically drain the water out of the separator bowl.

• Figure 2 shows the symbol used to designate a regulator. • The zigzag line coming out of the top of the symbol indicates the

output pressure is adjustable. • The horizontal arrow through the center of the square shows the

direction in which the air flows through the regulator. • The dotted line indicates a feed back. As air is used and the output

pressure starts to drop, the regulator will allow more air to flow through it, which will maintain the output pressure at the correct level.

• A gauge is almost always used with a regulator so we can adjust the regulator for the output pressure required.

• The symbol for a lubricator is shown in Figure 3. • Since the lubricator conditions the air it is also represented by a

diamond. • The straight line extending down from the top represents the tube

from which oil can be dropped into the air stream. • A drain is sometimes provided with a lubricator so that any water

that might accumulate in the bottom of the bowl can be removed. • Since water is heavier than oil it will accumulate on the bottom of

the bowl, beneath the oil, and can be drained without draining the oil.

• A complete filter, regulator and lubricator (FRL) symbol can become quite involved, as shown in Figure 4A.

• Figure 4B shows the simplified version of an FRL symbol.

Flow Controls

• A flow control is many times called a restrictor and is used to restrict or regulate the volume of air that flows through an air  line.

• There are basically two types of restrictors, fixed and variable or adjustable.

• If we were using a 1/4" air line and we placed a fitting in the air line that only had a 1/32" hole in it, this would be an example of a fixed restrictor.

• A variable restrictor can be adjusted to shut the air flow off completely, to allow for maximum air flow, or anywhere in between.

• Figure 5 shows some flow controls which are in common use today.

• These flow controls can be used for a variety of purposes.

• A few applications may include increasing or decreasing air flow for a needle cooler or guidance systems, increasing or decreasing the speed of an air cylinder, or reducing the impact of one device against another

• Figure 6 shows the symbol for a fixed restrictor and a variable restrictor.

• Another type of flow control is shown in Figure 7A. • This is a  variable restrictor with a check valve, sometimes referred to

as a speed control. • This valve restricts the air flow in one direction, but allows free flow in

the other direction. • Figure 7B shows the symbol used to designate this control.

• Referring to Figure 8A, as the air flows from right to left it pushes the check ball off its seat, allowing the air to flow freely through the check valve.

• In Figure 8B, however, as the  air flows from left to right it pushes the check ball into its seat, which prevents air from flowing through the check valve.  This causes all the air to flow through the variable restrictor

Directional Control Valves

• A directional control valve is a device which connects, disconnects or changes the direction of air flow in a circuit.

• The first thing that needs to be determined is the number of positions the valve has.

• Most valves have two positions, but some valves do have three positions. The number of positions a valve has is represented in its symbol by a series of squares.

• The symbol in Figure 1A is composed of two squares, which represent the two positions of this valve.

• The  symbol in Figure 1B is composed of three squares, which represent the three positions of this valve.

• It should be notedthat the squares can be drawn side by side or one on top of the other

• The second thing that needs to be determined is the number of ports the valve has.

• A port is an opening through which air can enter or exit a valve.

• The number of ports can be determined by examining the valve and counting them, or by looking at the valve's symbol.

• Figure 2 shows the symbol for a 2-position, 2-way directional control valve.

• It is a 2-position valve because it consists of two squares.

• It is a 2-way valve because if you look at any one square it has two ports labeled 1 and 2.

• Generally speaking, if each square has two ports it's a 2-way valve, three ports is a 3-way valve and four ports is a 4-way valve.

• There are, however, a couple of exceptions to this rule, which will be discussed later.

• In Figure 2, the bottom square shows ports 1 and 2 represented by a T symbol.

• This symbol is used to represent a port which is closed or blocked off.

• The top square shows ports 1 and 2 connected by a line with an arrow on it.

• The line is used to show that the two ports are connected.

• The arrow is added to one end of the line to indicate which direction the air flows through the valve.

• One square indicates how the ports are connected when the valve is off or de-energized; the other square indicates how the ports are connected when the valve is on or energized.

Actuators

• Before we can determine which square indicates the energized state and which square indicates the de-energized state, we must know the symbols for actuators.

• An actuator is the means by which the valve is energized and de-energized.

• Actuators are represented by symbols that are added to the ends of the directional control valve's symbol.

• Figure 3 shows some actuator symbols which represent manual operation.

• Figure 3A shows the symbol that is used to indicate that a pushbutton is depressed to energize the valve.

• Figure 3B shows the symbol that is used to indicate that a lever is operated to energize the valve.

• Figure 3C shows the widely used universal symbol to indicate manual operation. While this symbol indicates that the valve can be manually energized, it does not indicate the specific means such as, pushbutton, lever, foot pedal, etc.

• Figure 4A and 4B show some actuator symbols which represent mechanical operation.

• When a mechanical arm contacts the roller cam it pushes it down energizing the valve.

• Figure 4C shows the symbol used to indicate a spring. • Springs are normally used to return a valve to its rest position,

after it has been energized by another means.

• Figure 5A shows the symbol used to indicate electrical or solenoid operation. These valves are usually energized by a coil of wire called a solenoid.

• Figure 5B shows the symbol used to indicate an air operated valve. These valves are activated by pressure pushing on a diaphragm. These valves are commonly called pilot valves

• Sometimes valves can be energized by more than one actuator.

• In Figure 6, the symbol for an air pilot and a solenoid are stacked on top of each other.

• In this case it takes an electric solenoid and air pressure to energize the valve. If either one is missing the valve will not energize

• In Figure 7, the symbol for manual operation and an electric solenoid are next to each other.

• In this case the valve can be energized either by the electric solenoid or by some manual means, usually a pushbutton.

• Being able to activate the valve manually can be very desirable because it can make isolating a problem easier.

• Figure 8 shows the complete symbol for a 2-position, 2-way, solenoid-operated, spring-return directional control valve.

• Since the solenoid symbol is attached to the top square, the top square shows us that ports 1 and 2 are connected when the solenoid is energized.

• The spring symbol is attached to the bottom square. When the valve is de-energized, the spring will return it to its rest position. The bottom square shows us that ports 1 and 2 are disconnected when the valve is de-energized

1 2

1 2

• Figure 9 shows the complete symbol for another 2-position 2-way valve, but this one is manually or solenoid operated and spring returned.

• The symbols for valves can be drawn in any position that is convenient for the diagram.

• In Figure 9, the valve is drawn sideways.• Since the manual symbol and the solenoid symbol are attached to the

right square when the valve is energized, the right square shows us how the ports are connected.

• The spring symbol is attached to the left square. When the valve is de-energized the spring will return it to its rest position. The left square shows us that ports 1 and 2 are disconnected when the valve is de-energized.

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22

1

3-way directional control valves• Two-position, 3-way directional control valves

generally come in two different configurations, normally open and normally closed.

• Figure 1 shows the symbol for a normally open 2-position, 3-way, solenoid-activated, spring-return directional control valve.

• The two squares indicate this valve has two positions. • Since the actuator symbol for a solenoid is attached to

the top square, the top square shows us how the three ports are connected when the valve is activated.

• In its activated state, port 1 is blocked and port 2 is connected to port 3.

• Since the actuator symbol for a spring is attached to the bottom square, the bottom square shows us how the ports are connected when the valve is deactivated and the spring has returned it to its normal condition, with port 1 connected to port 2, and port 3 blocked.

• Since in its normal or deactivated state this valve allows air to go through it to an air cylinder, we say this valve is normally open.

2

2

• Figure 2 shows the symbol for a normally closed 2-position, 3-way, solenoid-activated, spring-return directional control valve.

• This valve is identical to the one in Figure 1 with one major exception; in its deactivated or normal position the in-coming air is blocked.

• It is important to remember that an open air valve allows air to flow through it, and a closed air valve blocks the flow of air.

• The terms open and closed have different meanings depending upon whether we are referring to pneumatics or electricity.

• Figures 3A and B show some typical 2-position, 3-way valves.

4-way directional control valves• Figure 4 shows the symbol for 2-position,

4-way, solenoid- operated, spring-return directional control valve.

• The two squares indicate this valve has two positions.

• Since the actuator symbol for a spring is attached to the bottom square, this square shows us how the four ports are connected when the valve is de-activated and the spring has returned it to its normal position, with port 1 connected to port 2, and port 3 connected to port 4.

• Since the actuator symbol for a solenoid is attached to the top square this  square shows us how the ports are connected when the solenoid is activated, with port 1 connected to port 3, and port 2 connected to port 4.

• Figure 5 shows the symbol for a 2-position, 4-way, 5-ported, solenoid-activated, spring-return directional control valve.

• Since the actuator symbol for a spring is attached to the left  square, this square shows us how the ports are connected when the valve is de-activated.

• The actuator symbol for a solenoid is attached to the right square so this square shows us how the ports are connected when the valve is activated.

• Note that even though this valve has five ports, one port is always blocked and only four ports are used at any one time

• Figure 6A and B show some typical 2-position, 4-way valves.

Air Cylinder

• An air cylinder is a device which converts pneumatic energy into mechanical energy.

• At this time we are concerned with two basic types of air cylinders: single- acting and double-acting cylinders.

• Figure 1 shows the symbol for a typical single-acting, single-rod spring-return air cylinder.

• It is single-acting because the air can only enter in one end; single-rod because the rod only extends out one end of the cylinder; and spring-return because a spring inside the cylinder will return the rod to its normal position when high pressure air to the cylinder is shut off.

• It should be noted that when the cylinder is activated the air in the spring side of the piston must be allowed to escape from the cylinder.

• For this reason a small hole is usually drilled through the side of the cylinder near the end to allow air to escape

• Figure 2 shows a typical single-acting air cylinder.

• Figure 3 shows the internal components of a single-acting, single-rod, spring-return air cylinder.

• Figure 4 shows the symbol for a double-acting, single- rod air cylinder.

• When high pressure air enters port B it will push the piston to the left, which will pull the cylinder rod into the cylinder.

• When the high pressure air is removed from port B and enters port A the piston will push the cylinder rod to the right, extending it.

• Figure 5 shows a typical double-acting, single-rod, air cylinder.

• Figure 6 shows the internal construction of a typical double-acting, single-rod air cylinder

• Figure 7 shows the symbol for a double-acting, double-rod air cylinder.

• It is double-acting because air can enter and be discharged from either end of the cylinder; and double-rod because the rod will extend out the left end of the cylinder when air enters port B, or out the right end of the cylinder when air enters port A.

• Figure 8 shows a typical double-acting, double-rod air cylinder.

• There are several factors which determine the amount of force an air cylinder can produce.

• For example, a large diameter cylinder will produce a greater force than a smaller diameter cylinder.

• Also, increasing the air pressure to the cylinder will increase the amount of force.

• The speed of an air cylinder is determined by the flow of air in cubic feet per minute (CFM) to the air cylinder.

• If we restrict the flow to a cylinder, it will move slowly.

• On the other hand, if we allow the air to flow to a cylinder unrestricted it will move with great speed,

• PRESSURE causes FORCE. FLOW causes SPEED.

• When regulating the speed of an air cylinder, the exhaust air coming out of the cylinder should be controlled rather than the air entering the cylinder.

• Controlling the exhaust air tends to give smoother and more consistent speed control of the air cylinder.

• In some applications of a single-acting air cylinder,  controlling the exhaust air will regulate the speed of the cylinder in only one direction.

• In these cases the input air must be controlled to regulate the speed of the cylinder in the other direction.